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Show 25 FUEL SAVINGS WITH OXYGEN ENRICHMENT FUEl: CH ... 2% EXCESS <>2 IN FLUE GAS AIR BASE CASE PREHEAT TEMP ("F) FLUE GAS TEMPERATURE (Oeg F) Figure 3 2500 3000 products preheat the incoming cold charge in a counter-current fashion in an unfired furnace zone before they exist through the furnace flue ports. Thus a portion of sensible heat of the combustion products is recovered by preheating the furnace charge. Since the volume of the combustion products is reduced and the bulk gas emissivity is increased with oxygen enrichment, a greater portion of sensible heat of the combustion products is recovered by preheating the charge resulting in higher energy efficiencies. The economics of oxygen enrichment for fuel savings is significantly influenced by the temperature drop of flue gases due to "countercurrent effects." An example is shown in Figure 4, where 50% of the current firing rate of the air burners are replaced with oxygen fuel burners or, equivalently, about 30% oxygen enriched air is used for all burners. Theoretical fuel savings with oxygen enrichment is about 9 MMBTUjton ° at a flue gas temperature of 20000 F. If th~ flue gas temperature is reduced by 3000F by the "counter-current effects," fuel savings are increased to about 15 MMBTUjton °2 , This results in an increase in break-even oxygen cost by as much as $24 at an energy cost of $4 per MMBTU . The actual extent of energy efficiency improvement by the "counter-current effects," depends on the furnace geometry of the unfired preheat zone, the extent of oxygen usage, and other heat transfer parameters. There is a diminishing return in specific fuel savings with higher levels of burner conversion to oxygen due to reduced flue gas temperature. The optimum conversion level, or oxygen enrichment level, must be analyzed for individual furnaces. Numerical furnace models have been successfully applied to predict the impact of oxygen enriched combustion for continuous steel reheating furnaces (Ref. 4,5). 155 6 ENHANCED FUEL SAVINGS IN "COUNTER CURRENT" FURNACES BASIS: 50% CONVERSION 1600 1600 2000 2200 2400 Flue Temperature (Deg F) Figure 4 PRODUCTIVITY IMPROVEMENTS Oxygen enrichment has been successfully used for productivity increases in a broad range of industrial furnaces listed in Table 1. Throughput increases of 10-20% are typically possible for most furnaces with various furnace limitations shown in Table 2. Higher available heat and small flue gas volume associated with oxygen enrichment are very effective in overcoming the capacity limitations of fuel and air supply systems and the flue handling system. More than 40% increases in productivity have been achieved in certain process furnaces including glass melting, and aluminum remelting furnaces. Dust carryover problems in cement, lime and other rotary kilns have also been effectively alleviated by the smaller flue volumes resulting from oxygen enrichment. High temperature oxygen or oxygen enriched flames have been successfully applied to certain glass mel ters and kilns to increase heat transfer to strategic areas in a furnace. In most furnaces, however, uniform temperature distribution is a critical requirement and high temperature flames are often serious concerns to furnace refractory walls. Contrary to the conventional wisdom, a higher temperature flame is not necessarily a requirement to achieve a higher heat transfer rate. Gas radiation from hot combustion products to the surrounding refractory walls and re-radiation to the heat load is the primary mode of heat transfer in most high temperature furnaces. The intensity of gas radiation is not only a function of gas temperature and concentrations of CO 2 , H 2 0, and soot, but also a strong function of the volume of the radiating gas (i. e. beam length). For example, calculations show that the radiative heat flux from combustion products at 25000F in a 6 foot-diameter sphere is greater |